198 Godwin- Austen— Cretaceous Rocks. 
Cretaceous series of the Netherlands; so that the O. auricularis is 
alone available. This is scarce as British; and if, as the observa- 
tions of Morris and D’Orbigny would lead us to suppose, this species 
should be the O. halitoidea and O. Rauliniana of the Gault of 
France, as of the Gault and Upper Greensand of England, present- 
ing modifications of form according to its duration in time and con- 
ditions of life-zone, it can hardly serve the purpose of a ‘ Leit-Mus- 
chel’ or ‘ Guide- Shell.’ 
The third horizon contains Ammonites peramplus, Am. papalis, 
Am. Deverie, and Radiolites cornu pastoris. The second of these 
is a Mediterranean-area species, Am. Deverie: only one English 
specimen has been reported ; and the characteristics which distin- 
guish it from Am. Rothomagensis are very unimportant. Am. per- 
amplus in this country ranges from the Lower Chalk into that 
without and with flints, through what must represent a vast space 
of time. Aadiolites cornu-pastoris occurs on the Atlantic side of 
France, but 5° of latitude south of our Chalk area. 
Division fourth has four typical fossils—IJnoceramus labiatus, 
Ammonites Weilhancit, Hemiaster Verneuilli, and Rhynchonella 
Cuviert. Mr. Sharpe puts this Ammonite doubtfully as the Am. 
Cenomanensis of D’ Archiac, a shell from the Grey Chalk of the South- 
east of England ; but it has been very reasonably suggested that 
Am. Cenomanensis may be only a variety of Am. Rothomagensis, and 
of the extreme stage of its growth. ‘The range of the Hemiaster is 
far south of our area. The Jnoceramus and Rhynchonella have a 
long range in the Lower Chalk and the Sands next below. 
In the next division—fifth—there are two British Ostree, and an 
Ammonite—Am. navicularis, which is perhaps the commonest form 
in the Lower Chalk of the North Downs. With it occurs Ostrea 
carinata-frons ; but this shell is much more abundant in our sandy 
beds below the Chalk. Dr. Reynés places Ostrea Columba at the 
head of the series. In Dorset the place of this shell is far below the 
Chalk. M. D’Orbigny remarks that in the North of France the 
position of this species is below the zone of Am. Rothomagensis, which, 
so far as our limited data go, holds good for this country also. 
The sixth horizon is that with Turrilites costatus and T. tubercu- 
latus, Ammonites Rothomagensis and Am. inflatus-rostratus. Here 
again species are put together which, so far as the English South- 
east area is concerned, might well stand apart. 7’. costatus is abun- 
dant in the Grey Chalk, and 7. tuberculatus has its maximum in 
the lower arenaceous strata, with phosphatic nodules: such is also 
the position of Am. inflatus. 
M. D’Orbigny states that in the North of France the position of 
Am. navicularis is always lower than that of Am. Rothomagensis. 
Dr. Reynés reverses this order, and puts them into distinet subdivi- 
sions. Should both observers be correct, it is a circumstance worth 
attending to, as showing a different distribution in different pro- 
vinces. 
These six ‘horizons’ include the whole of the Chalk of England; 
and, viewed in a general way, the thirteen forms cited may occur . 
